The most common solder materials are tin/lead solders. These solders have been used to join metals for at least a millennium, and they have been the best material for the interconnection of modern electronic components and packages during the past several decades. The prevalent use of tin/lead solders is due mainly to their low cost and unique combination of material properties. McCormack, M. et al., Proc. IEEE/CPMT Int'l Electronics Manufacturing Technology Symp. pp. 7-14 (1994).
However in light of recent environmental and health concerns, there have been increasing research efforts to develop lead-free solders to replace the tin/lead solders. Shangguan, D. et al., Proc. IEEE/CPMT Int'l Electronics Manufacturing Technology Symp., pp. 25-37 (1994). Lead and lead-bearing compounds are among the most toxic substances, and the environmental and health issues concerning the toxicity of lead threaten to ban the use of lead-bearing solders in electronic applications. These concerns have inspired a great deal of research effort in the development of suitable lead-free solder alloys.
The potential candidates of lead-free solders which have been investigated are eutectic Sn/Bi, eutectic Sn/Ag, eutectic Sn/Zn, and eutectic Sn/In. Even though some lead-free solders have been in use in special applications for several years, there is no suitable substitute for tin/lead solder alloys at present.
Miniaturization in the microelectronics industry and the widespread use of surface mount technology (SMT) which was introduced in the 1960's, have also demanded higher performance of solders. Solders have to provide a structural function by mechanically supporting the electronic devices in addition to playing a role as the electronic contacts between printed circuit substrates and surface mount devices. Lau, J. H. et al., Solid State Tech. pp. 91-104 (1985). The mechanical properties of solders have also become more important because of the small size of solder joints in high density surface mount device packages. The development of alternative lead-free solders with superior mechanical properties to tin/lead solder leading to improved reliability of solder joints, is thus desirable.
The service temperature for solders are generally high relative to their absolute melting point and solder microstructures tend to be highly unstable under typical operating conditions. Solder materials are also readily subject to thermal stresses induced by the thermal expansion mismatch between components and substrates during thermal cycling. Consequently, the development of alternative solder materials possessing stable microstructure and improved mechanical strength to ensure reliability of solder joints is also desirable.
Approaches to strengthen the conventional solder materials by introducing fine second phase particles to solder to create a composite have been under investigation. Marshall, J. L. et al., Proc. Conf. NEPCON., West Anaheim, Calif., pp. 1278-1283 (1992); Betrabet, H. S. et al., Script Metall. 25:2323-2328 (1991); Sastry, S. M. L., et al., Proc. Conf. NEPCON, West Anaheim, Calif., pp. 1266-1275 (1992); Clough, R. B. et al., Proc. Conf. NEPCON., West Anaheim, Calif., pp. 1256-1265 (1992); Pinizzotto, R. F. et al., Proc. Conf. NEPCON., West Anaheim, Calif., pp. 1284-1298 (1992); Betrabet, H. S. et al., Proc. Conf. NEPCON., West Anaheim, Calif., pp. 1276-1277 (1992); Kuo, C. G. et al., 1st Int'l. Conf. Microstructures and Mechanical Properties of Aging Materials, ed. P. K. Liaw, R. Viswanathm, K. L. Murty, E. P. Simonen and D. R. Frear, The Minerals Metals & Materials Society, TMS, Warrendale, Pa., pp. 417-423 (1993); Kuo, C. G. et al., 1st Int'l. Conf. Microstructures and Mechanical Properties of Aging Materials, ed. P. K. Liaw, R. Viswanathm, K. L. Murty, E. P. Simonen and D. R. Frear, The Minerals Metals & Materials Society, TMS, Warrendale, Pa., pp. 409-415 (1993). Such particles may be introduced either by precipitation from supersaturated solids or liquid solution or by external addition of foreign particles as dispersoids. The precipitation strengthening may not be a suitable method because the precipitates are generally prone to coarsening unless one or more of the constituent elements in the precipitate phase are chosen to have low solubility and diffusivity in the solder matrix. On the other hand, foreign dispersoid particles introduced within solder alloys would not coarsen easily since the elements or compounds involved can be chosen to have low solubilities and diffusivities or no reactivity with the matrix solder alloy. McCormack, M. et al., IEEE Trans. Comp. Hybrids Manuf. Tech.—Part A 17(3):452-457 (1994). Mechanical methods such as vigorous shearing while the composite solder is in a semi-solid phase have also been used to produce intermetallic phases with small homogeneous particles (U.S. Pat. No. 5,094,700).
Several types of particles have been used to produce composite solder with stable microstructure and improved mechanical properties. The representative particles used are the powder of intermetallic compounds (Cu6Sn5, Cu3Sn), elemental metals such as Cu, Ni, Ag, Au to introduce intermetallic compounds with Sn in the solder matrix, aluminum oxide (Jin, S., Final Program ASM-TMS Materials Week '96, ASM International and The Minerals, Metals & Materials Society, Cincinnati, Ohio, pp. 116 (1996)), and carbon fiber.
The intermetallic phase dispersoids should be small enough and closely spaced to yield significant strengthening without considerable loss in ductility. It is also important to uniformly distribute small dispersoid particles in sufficient quantity and maintain this dispersion after the alloy is melted and reflowed to positively affect mechanical properties of the solid solder. A magnetic field was applied to molten solders containing fine Fe particles to overcome the gravity-induced particle segregation problem and to achieve a microstructure with a uniformly distributed three-dimensional network of the dispersoid particles. McCormack, M. et al., IEEE Trans. Comp. Hybrids Manuf. Tech.—Part A 17(3):452-457 (1994).
The dispersoids should make composite solder more resistant to creep-fatigue deformations by acting as heterogeneous nucleation sites to produce a very fine, initial microstructure of solder matrix upon reflow, and by serving as obstacles to grain growth, crack growth, and microstructural coarsening to retain the stable microstructure of solder even after thermal exposure. Betrabet, H. S. et al., Script Metall. 25:2323-2328 (1991); Wasynczuk, J. A. et al., Proc. Conf. NEPCON., West Anaheim, Calif., pp. 1245-1255 (1992); Marshall, J. L. et al., IEEE Trans. Comp. Hybrids Manuf. Tech. 14(4):698-702 (1991).
There is also a need for a solder with a low coefficient of thermal expansion, since the stresses are thermally induced in solder joints due to the difference in thermal expansion coefficient between substrate and solder. Carbon fiber with nearly zero thermal expansion coefficient as a dispersoid was developed to adjust the thermal expansion coefficient of solder. Composite solders with the amount of carbon fiber which makes thermal expansion coefficient of solder to be similar to that of substrate, exhibited prolonged fatigue lives. Ho, C. T. et al., J. Mater. Res. 5(6):1266-1270 (1990).
Solder joints in electronic modules undergo temperature cycling which is caused by power on/off cycles, daily temperature variations, and seasonal changes. Solder joints are subject to a cyclic loading condition imposed by thermal stresses induced by the difference between the thermal expansion of electronic components. Since the thermal cycling ordinarily exhibits a low strain-rate, solder joints basically undergo low frequency thermal fatigue. The service temperature of solder joints corresponding to high homologous temperature and relatively long hold times are often encountered during service conditions. Thus time-dependent, thermally-activated deformation or creep becomes significant at high homologous temperature. Thermal aging and cycling lead to heterogeneous coarsening that accelerates the nucleation of cracks. Frear, D. R. et al., JOM, pgs. 18-22 (June, 1988). Consequently, damage or failure of solder joints is caused by creep-fatigue interaction which typically describes the cyclic application of a load at temperatures where time dependent, thermally-activated processes are significant.
The creep-fatigue damage in solder joints has been identified in the analysis of fracture surface of solder joints which were thermally cycled, where fatigue striations were observed side by side with creep voids and extensive intergranular cracking. Attarwala, A. I. et al., J. Electron. Packag. 114:109-111 (1992). The eutectic tin/lead solder subjected to isothermal cyclic loading at room temperature showed that deformation occurred primarily by creep process and was very sensitive to hold time or frequency resulting in increasing creep rate and decreasing cycles to failure with increasing hold time. Weinbel, R. C. et al., J. Mater. Sci. 22:3901-3906 (1987); Tien, J. K. et al., IEEE Trans. Comp. Hybrids Manuf. Tech. 12(4):502-505 (1989). Creep was also observed to be the predominant mode of deformation for isothermal fatigue in which good correlation was found between the isothermal fatigue life of solder joints and the amount of creep strain per cycle. Shine, M. C. et al., ASTM STP 942:588-610 (1988).
In summary, the reliability concerns of solder joints in addition to the environmental concerns in the use of toxic materials, place greater demands on the performance of solder joints especially in electronic applications. It would thus be desirable to provide an improved solder which meets these demands. It would also be desirable to provide an improved solder which is lead-free. It would further be desirable to provide lead-free composite solders incorporating particulate reinforcement that enhances the solder's resistance to thermal fatigue damage. It would also be desirable to provide a method for producing lead-free, in-situ composite solders which yield solder joints with improved reliability, especially in electrical systems. It would yet further be desirable to provide a composite lead-free, in-situ solder having a microstructure such that a solder joint employing the solder would have improved resistance to creep and thermal fatigue damage.